Response Latency and Self-Directed Behaviours by Capuchin Monkeys in an Intertemporal Choice Task

Several studies have investigated response latency in different tasks, showing that more complex choices usually require more time than less complex ones. In both humans and non-human primates self-directed behaviours, such as scratching, are related to frustration and anxiety. A few studies demonstrated that in great apes the occurrence of self-directed behaviours during cognitive tasks increase with task difficulty. Here, we investigated how response latency, scratching and other potentially stress-related behaviours vary in nine capuchins faced with an intertemporal choice task, an inhibition task where subjects face choices between a small immediate option and a large delayed option. We scored: (i) response latency, (ii) scratching, alarm calls, and pointing at chosen/not chosen option (a behaviour indicating motor impulsivity) during the delay associated to the large option, and (iii) scratching and alarm calls during the intertrial interval. Overall, there was a significant decrease of response latency across sessions. Moreover, in the course of the study, capuchins adjusted their behaviour to the hard task requirement (waiting for a desired reward), becoming less stressed during the delay. In fact, (i) scratching decreased both within each session and across sessions, and (ii) pointing at chosen/not chosen option decreased across sessions. In contrast, during the intertrial interval alarm calls increased across sessions. Thus, experience helps capuchins to cope with productive delays (i.e. instrumental to obtaining a reward), but has a negative impact on their tolerance for meaningless delays (i.e. non related with the specific task, as in the intertrial interval).